Film packaging is commonly used today for many different products, such as food products. During the packaging of such products, the film layers of the product packaging are usually sealed together in some manner to close off the film packaging so that the package contents are effectively sealed within the film. The films used for such packaging are often formed from thermoplastic polyolefin resins, such as polypropylene. One means of sealing such materials is through the use of heat-seal films, layers or coatings. The heat-seal films are usually comprised of thermoplastic layers of resin that are coextruded or otherwise joined to a core layer or layers of the polymer film of the product packaging. The core layer(s) have a higher melting point than the heat-seal layer. Thus, when heat and pressure are applied to layers of the packaging film, the heat-seal layer is melted so that it forms a bond or seal to seal the layers of film together.
Heat-seal films are often characterized in terms of seal initiation temperature (SIT) and heat-seal strength. In commercial production of film packaging utilizing heat-seal films or layers, it is often desirable to reduce the amount of heat necessary to provide an effective seal without compromising the seal strength or integrity. By reducing the amount of heat necessary to seal film packaging, savings in energy as well as improvements in line speed and processability can be achieved.
Materials traditionally used for heat-seal films include polypropylene homopolymers, ethylene-propylene copolymers and ethylene-butene-propylene terpolymers formed from Ziegler-Natta catalysts. Ziegler-Natta catalysts are used in forming stereospecific polymers, particularly isotactic polymers. Isotactic polypropylene is one of a number of semi-crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. The structure of isotactic polypropylene is characterized in terms of the methyl groups attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main polymer chain. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following formula:

As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity or crystallinity. This crystallinity distinguishes isotactic polymers from atactic polymers in which the methyl groups are randomly oriented along either side of the polymer chain and are non-crystalline and highly soluble in xylene. Although isotactic polypropylene is generally insoluble in xylene, a certain amount of polymer, which may be the result of chain defects within the polymer chain and low molecular weight chains, may be soluble in xylene, including those prepared with conventional Ziegler-Natta catalysts. In many instances, particularly in packaging used for food products, a high percentage of xylene solubles is undesirable and can even make the film unusable for certain applications.
Ziegler-Natta propylene-ethylene random copolymers have been and continue to be used in large volumes to produce heat seal films. Although these materials typically form films with good seal properties, good optical properties and comply with appropriate FDA guidelines, improvements are still desired. One characteristic associated with conventional Ziegler-Natta propylene-ethylene copolymers is that during the polymerization of such materials, the ethylene monomers are frequently inserted into the polymer chain in large ethylene groups or blocks instead of being inserted and distributed in a regular manner throughout the polymer chain as smaller segments or blocks. The presence of these larger ethylene blocks or segments in the polymer chain, especially in the lower molecular weight and/or atactic fraction, results in less efficient use of the comonomer and leads to higher levels of xylene solubles.
Heat-seal films having improved heat-seal characteristics, as well as other improved properties, such as low xylene solubles, are therefore desirable. It is therefore an object of the present invention to provide heat-seal films having such improved properties, as well as overcoming many of the disadvantages associated with prior art heat-seal films.